Can't update/install any non apple software

Recently I've started using my Macbook pro again after leaving in hibernation mode for about a year(don't ask!). Naturally there were a ton of software updates not just from apple but from browsers and adobe etc. Apart from all the apple software(including the Mac OS X combined update), which ran successfully the second try(first attempt had some corruption issue, I had to re-download everything and install again), I can't update or install anything else. This includes applications like Firefox, Seamonkey and clamxav antivirus. I get the same error message for all of them:- "The operation can’t be completed because some items had to be skipped. For each item, choose File > Get Info, make sure “Locked” is deselected, and then check the Sharing & Permissions section. When you are sure the items are unlocked and not designated as Read Only or No Access, try again."

I've checked the permissions and it is in fact set to read and write. I've tried it in both the master account and the regular admin account I use. I've also ran disk permissions repair, but there's still something that shows up as "differing" from what it should be when I run verify disk permissions no matter how many times I've repaired it. Something to do with the language support in system/library/preferences.

Please help as I really do not wish to have to do anything drastic like reinstalling the OS etc.

Helps if you provide operating system otherwise we are guessing. However either boot from OS X.6 DVD, or Lion Recovery depending on the version, and run Repair Disk and see what is reported. If all well and good run Repair Permissions. Next suggestions is a clean install.

Hang on to those original install discs like grim death! Using OS X.7 or later make a bootable USB thumb drive before running Installer!